1. Field of the Invention
The present invention relates to a data storage technology and, more particularly, a virtual tape library (VTL) backup system, and a method of hard disk drive (HDD) power control in VTL devices.
2. Description of the Related Art
As a hard disk storage device, the VTL device comprises a storage controller (SC) and a Redundant Array of Independent Disks (RAID).
In the VTL device, the hard disk drives (HDDs) in the RAID array are the storage media; the SC comprises a RAID subsystem, which, serving as the RAID management module, handles the read/write operations on the data stored on the HDDs and provides redundant protection for data; the VTL application program configured on the SC shows the HDDs in the RAID array as virtual tapes to clients assigned to the VTL device.
Currently, VTL devices are often used to back up data. The clients of VTL devices are usually backup servers. A VTL device and a backup server together form a VTL backup system.
As shown in FIG. 1, in a VTL backup system, the backup server that serves as the client and the VTL device are connected through an Internet Protocol (IP) or Fiber Channel (FC) network and communicate with each other using the Internet SCSI (iSCSI) or FC protocol.
The backup server comprises a backup application module (BAM), which is an application program running on the backup server. When the BAM performs a backup operation, the backup data is written from the backup server to the storage media in the VTL device through an IP or FC network; when the BAM performs a disaster recovery (DR) operation, the data previously backed up is read from the VTL device and written back to the backup server through the IP or FC network.
As shown above, in a VTL backup system, data read/write operations occur in the VTL device only when the BAM performs backup/DR operations. In other words, the HDDs in the VTL device are idle for most of the time. Idle HDDs staying in the power-on state cause unnecessary power consumption of the VTL device.
To cut down power consumption, the Massive Arrays of Idle Disks (MAID) technology is often used to power on or power off specific HDDs, instead of all HDDs, in the RAID arrays of VTL devices.
To implement the MAID technology in a VTL backup system, you need to configure a few rotating HDDs in the VTL device as a cache. When the RAID subsystem in the SC receives a write or read request from the backup server, it checks whether the data required by the backup server is stored in the cache. If not, it powers on the HDD(s) corresponding to the write or read request, and records the access status of the HDD(s). On the other hand, if a power-on HDD stays in the idle state for a certain period of time, the RAID subsystem powers it off based on its recorded access status and the preconfigured power control policies. In other words, in a VTL device, only HDDs involved in ongoing write or read operations are powered on, while the rest of the HDDs stay in the power-off state. In this way, power consumption of the VTL device is lowered.
Although the use of the MAID technology lowers the power consumption of VTL devices, a few drawbacks do exist in the implementation of the technology:                1) The RAID subsystem in the VTL device needs to record the access status of each HDD in real time, which consumes a large amount of system resources in the VTL device;        2) The RAID subsystem in the VTL device needs to precisely identify the HDD corresponding to the virtual tape that is being accessed and control its power-on and power-off states. This poses high requirements on the reliability of the RAID subsystem and involves complex implementation procedures;        3) Because the HDDs in the VTL device are idle for most of the time, the cache usage is lower. Configuring a few HDDs as a cache is actually a waste of hardware resources;        4) Due to the inherent attributes of HDDs, powering on an HDD always requires a considerable amount of time. For example, it takes 10 seconds to power on a Serial Advanced Technology Attachment (SATA) HDD. Therefore, if the BAM needs to wait for the HDD(s) to be powered on after it already starts a backup/DR operation, the overall performance of the VTL backup system will be compromised.        
It can be concluded thereby that using the MAID technology to cut down the power consumption of a VTL device affects the overall performance of the VTL backup system, and consumes a large amount of system resources.